namespaced validation logs to prevent overwriting

[eantono]

Motivation

The validation loss is currently not logged in the setting of both train and val workflow modes.

The problem is described in this issue:
open-mmlab/mmsegmentation#1396
This PR replicates the changes made in this PR to the mmsegmentation library: open-mmlab/mmsegmentation#1494

Modification

val_step is modified to add _val suffix to all validation losses, to distinguish those losses from those in the training step. This enables both training and validation loss logging.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Erin Antono seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}